In CDMA radio access networks, Walsh codes and QoF codes are assigned to each user for baseband signal spreading. The IS2000 specification defines one Walsh code set and four QoF code sets. Each set has 128 Walsh/QoF codes, each with a length of 128 bits. Each user is assigned a different Walsh code or QoF code.
In CDMA Single-Carrier Radio Transmission Technology (1×RTT), a Walsh code allocation algorithm as defined in U.S. Pat. No. 7,539,162 has been implemented in Base Transceiver Station/Base Station Controller (BTS/BSC) products. This allocation algorithm enables a BTS to support approximately 35 Enhanced Variable Rate Codec (EVRC) users and 46 advanced EVRC (EVRC-B) users in one carrier-sector. In this case, BTS user capacity is limited by forward power, and 128 Walsh codes are sufficient to support maximum BTS user capacity. QoF codes are not used, and therefore no QoF allocation algorithm has been implemented in BTS products.
Another reason that QoF codes have not been used is that their use introduces significant non-orthogonal interference. High non-orthogonal interference causes a mobile terminal that uses a QoF code to experience much higher interference than mobile terminals using Walsh codes. This results in a negative service quality impact.
With the recent introduction of 1×Advanced carrier, user capacity for one carrier increased to approximately three times the capacity for a 1×RTT carrier. As a result, the Walsh code set is no longer sufficient, and allocating mobile terminals with a QoF code is necessary. As a part of 1×Advanced technology, new mobile terminals may support Advanced Quasi-Linear Interference Cancellation (AQLIC), which maximally cancels non-orthogonal interference.